The reaction is catalysed by a component of EC 2.4.1.22 (lactose synthase), which is identical with EC 2.4.1.38 (β-N-acetylglucosaminyl-glycopeptide β-1,4-galactosyltransferase), and by an enzyme from the Golgi apparatus of animal tissues. Formerly listed also as EC 2.4.1.98.